During the combustion of coal, and in particular combustion of many lower grades of coal, sulfur oxides are produced which, if emitted into the atmosphere can cause significant environmental pollution. Several methods for reducing the amount of sulfur oxide emissions that are released into the atmosphere have been developed.
One method is to burn low sulfur coal in an attempt to avoid problems associated with excessive sulfur oxide emissions. However, such fuel is not always readily available and the cost to transport such high quality coal is in many cases prohibitive. Significant deposits of high sulfur coal exist in the United States, predominantly in the Eastern part of the nation. Therefore, significant research has been conducted and several processes developed for reducing emissions from such high sulfur coals.
It is known that, under certain conditions, calcium reacts with sulfur oxides in flue gas from the combustion of sulfur-containing coal to capture sulfur as calcium sulfite and/or calcium sulfate, thereby reducing the release of sulfur oxide emissions.
It is also known that, calcium-containing sorbents can be added directly to the combustion chamber to react with sulfur oxides in the high temperature region following combustion. U.S. Pat. No. 4,824,441 by Kindig, issued Apr. 25, 1989 discuss several methods that have been tried to improve the sulfur capturing capability of calcium-containing sorbents in the high temperature region following combustion. For example, various promoters and catalysts can effect the efficiency of sulfur capture. Also, mixing the sorbent material with the coal prior to combustion increases the residence time of calcium with sulfur oxides in the high temperature region in which calcium is reactive with sulfur oxides.
It is also known that calcium will react with sulfur oxides in a lower temperature environment, wherein the temperature is close to the temperature at which water, and generally sulfuric acid produced by water and sulfur oxides, condense from the flue gas stream. Such condensation temperature is also sometimes referred to as the saturation temperature or the adiabatic saturation temperature.
U.S. Pat. No. 4,867,955 by Johnson, issued Sep. 19, 1989, discusses the injection of calcium-containing sorbent and water into the flue gas to promote reaction of calcium with sulfur oxides to effect capture of sulfur in a lower temperature, humidified environment. U.S. Pat. No. 4,867,955 also discusses injection of calcium carbonate into the combustion chamber to effect calcination of the calcium carbonate to calcium oxide. Such calcined sorbent is thereafter reacted in a lower temperature, high humidity environment to effect sulfur capture.
Some attempts have been made to combine sulfur capture in the high temperature combustion region with sulfur capture in a subsequent lower temperature, high humidity environment. U.S. Pat. No. 4,519,995 by Schrofelbauer et al., issued May 28, 1985 discusses a process wherein lignite and calcium carbonate are pulverized and fed together to coal dust burners. Some flue gas is recycled to the combustion zone to reduce the temperature in that region. Prior to entry of flue gas into the dust filter, the relative moisture of the flue gas is increased by cooling the hot flue gas in a heat exchanger and/or by injecting or spraying water into the hot flue gas such that additional sulfur is captured in a lower temperature, humidified environment by sorbent that has collected on the bag filter surface. U.S. Pat. No. 5,002,743 by Kokkonen et al., issued Mar. 26, 1991, discusses the addition of sorbent to the combustion zone, independent of the fuel, followed by humidification to effect reaction at a lower temperature.
As these references indicate, there is a need to efficiently reduce environmental pollution by sulfur oxide emissions following combustion of coal. More efficient and less costly sulfur removal techniques are required to effectively use existing high sulfur coal resources.
The process of the present invention involves capturing sulfur in a high temperature region following combustion of coal and also in a lower temperature, high humidity environment under conditions such that the efficiency of sulfur capture is enhanced and the need for expensive modifications of existing processes and equipment is reduced.